1. Field of the Invention
This invention relates to transistors with improved output tuning and isolation.
2. History of the Art
High frequency power transistors typically utilize elongated collector and base electrodes connected to the collector and base contacts, respectively, of a plurality of constituent transistor cells. For one example of the structure of such transistors, see applicant's co-pending application, Ser. No. 849,302, filed Nov. 7, 1977, and assigned to applicant's assignee.
Various problems associated with impedance matching have prevented full realization of the potential power output from such devices. Microwave system requirements dictate that at useful frequencies and power levels a transistor package exhibit a predetermined output resistance, typically on the order of 50 ohms. This requirement means that the transistor resistance, often so low as 5 ohms, must be raised to a 50 ohm output and, for operation at a reasonable range of frequencies, the output capacitance (C.sub.ob) must be tuned out.
One prior art method for tuning out the output capacitance involves disposing a pair of DC blocking capacitors at either end of the collector and introducing a shunt inductance through the disposition of bonding wires connecting the collector end regions through the respective blocking capacitors to the base.
The disadvantage of this approach, however, is that it fails to provide optimum tuning for all portions of the transistor. It provides different effective inductances to different regions of the elongated collector with consequent variation of the output resonance frequency along the collector. The result in an undue limitation on the power levels available at high frequencies.
Another prior art arrangement avoids this problem by providing distributed shunt inductance tuning connecting the base to the output line through a blocking capacitor. This result, however, is achieved at the expense of destroying isolation between the transistor input and output. As a consequence of this connection to base, the base can be connected to ground only on the input side with the consequence that both input and output signals flow through the base-to-ground connection. The result is the generation of feedback signals which can displace the gain from its optimum level and distort data signals.
Accordingly, there is a need for a microwave transistor with isolated input and output and distributed output shunt tuning.